1. Field of the Invention
The present invention relates to a magnetic resonance imaging apparatus of the type having a magnet with at least one patient receptacle and at least one support plate, as well as a predetermined number of transmit and/or receive coils, and which allows exposures with the subject in at least two predetermined exposure positions, with at least one exposure taking place using predetermined adjustment parameters.
2. Description of the Prior Art
In known magnetic resonance imaging apparatuses, for different exposure positions the associated exposure parameters must in general be modified at least partially. The exposure parameters that may have to be modified include e.g. the position of the support plate and the connection or disconnection of transmit and/or receive coils. As a rule, a modification of the exposure parameters requires a readjustment of the magnetic resonance imaging apparatus.
The adjustment essentially serves to optimize the RF chain (transmit and receive coils and associated amplifiers) and to optimize the homogeneity of the main magnetic field produced by the magnet (also called the basic magnetic field or B0 field) in a volume under examination (imaging volume) located inside the patient receptacle. Due to the necessary homogeneity of the examination volume, this volume is also called the homogeneity volume.
The exposure parameters are also patient-dependent, since the patient represents an attenuation or damping for the transmit and/or receive coils. A precise adjustment thus also serves for patient safety with respect to the RF exposure.
In general, known magnetic resonance imaging apparatuses recognize automatically whether the. exposure parameters belonging. to particular exposure positions must be modified, and carry out a readjustment if warranted.
This adjustment normally requires 10 to 90 seconds per exposure position. Given certain examination procedures, this time is not available. This includes e.g. the tracking of doses of contrast agent over a larger body region that exceeds the available homogeneity volume of the nuclear spin resonance apparatus. In such cases, the patient must be guided by displacement of the support plate in a manner corresponding to the flow of contrast agent. If a smaller viewing field is not acceptable, the readjustment that is thereby required per imaging measurement (exposure) requires a multiple dosage of contrast agent, which is not desirable for the patient.
Alternatively to a smaller viewing field or to multiple injections of contrast agent, it is possible after the first adjustment to omit the further adjustments (readjustments) inherently required for high-contrast exposures. However, this leads to a considerable worsening of the image quality.
An object of the present invention is to provide magnetic resonance imaging apparatus of the type described above that provides high-contrast exposures in a short time, even given an examination of larger body segments.
This object is achieved in accordance with the principles of the present invention in a magnetic resonance imaging apparatus having a magnet and at least one patient receptacle and at least one support plate, as well as a predetermined number of transmit and/or receive coils. At least in two predetermined exposure positions, an exposure respectively takes place using predetermined adjustment parameters. The required adjustment parameters are inventively determined in a preceding adjustment process, and the exposures are executed in a subsequent exposure process.
For example, the exposure parameters can be modified by means of a spatial modification of position (longitudinal displacement, transverse displacement, rotation) of the support plate within the patient receptacle. Alternatively, or in addition, a modification of the adjustment parameters can take place by connection and/or disconnection of the transmit coils and/or the receive coils.
In the inventive magnetic resonance imaging apparatus, the required adjustment parameters are not determined immediately before each individual exposure, as is conventional. Rather, the required adjustment parameters are determined in an adjustment process that precedes the exposure process. Only after the determination of the required adjustment parameters are the exposures carried out, in a separate imaging exposure process.
The adjustment parameters are of course stored at least until the conclusion of the examination. The adjustment parameters thus can be used again, when identical or suitably similar exposure parameters (position of the support plate and configuration of the transmit and/or receive coils) are again reached in the context of the same examination.
In examinations with the inventive apparatus, high-contrast exposures are thus obtained, since it is not necessary to omit an adjustment. Due to the fact that the adjustment is carried out in a separate adjustment process, and the adjustment parameters are stored until the conclusion of the examination, the transmit and receive coils, or their coil elements, can be switched quickly during the examination, so that, in addition, reduced examination times result.
The inventive solution is suitable for a large number of different forms of magnetic resonance imaging apparatuses. Thus, for example, the magnet can be fashioned as a cylindrical magnet (solenoid) or as a horseshoe magnet (C-arm apparatus). Given cylindrically shaped magnets, the patient receptacle is fashioned as a patient tube.